X-ray imaging and tomography Giuliana Tromba Elettra - Sincrotrone - PowerPoint PPT Presentation

X-ray imaging and tomography Giuliana Tromba Elettra - Sincrotrone Trieste SY nchrotron R adiation for ME dical P hysics (SYRMEP) beamline School on Synchrotron and Free-Electron-Laser Methods for Multidisciplinary Applications, ICTP – Miramare, 7-18 May 2018 1 ICTP School 2018 Giuliana Tromba

Outline Characteristics and potentials of synchrotron X-rays SR X-rays imaging techniques Absorption, K-edge imaging Phase contrast techniques: Propagation Based Imaging (PBI) Analizer Based Imaging (ABI) X-ray interferometry with crystals Grating interferometric imaging (GI) Grating non-interferometric imaging Applications Biomedical imaging and biology Cultural Heritage and Paleoantropology Volcanology 2 ICTP School 2018 Giuliana Tromba

Advantages of SR for hard X-ray imaging Monochromaticity allows for: - optimization of X-ray energy according to the specific case under study (dose reduction) - quantitative CT evaluations - no beam hardening - convenient use of contrast agent (K-edge and L-edge imaging) Spatial coherence enables the applications of phase sensitive imaging techniques - Phase contrast overcomes the limitation of conventional radiology PHC image - It brings to a dose reduction - Improved contrast resolution, edges enhancement - Use of phase retrieval algorithms High fluxes - Short exposure time - Dynamic studies (4DCT) Collimation - parallel beams, scatter reduction - beam shaping (micro-beams) Absorption image ICTP School 2018 Giuliana Tromba

SR X-rays imaging techniques K-edge subtraction imaging Exploiting the monochromaticity of SR… 4 ICTP School 2018 Giuliana Tromba

K-edge Subtraction Imaging 1. Contrast agent: Iodine, or Gadolinium, etc. 2. Two Images are acquired : Above (A) and Below (B) the K-edge of Contrast agent 3. From image processing : Iodine and Tissue images can be obtained  xy : Attenuation coefficients : x = energies (A or B), Energy E K- (B) y = material (tissue (t) or iodine (i)). Energy E K+ (A) 100.0         Attenuation coefficient (cm 2 /g) ln A ln B I x Bi Ai      i Gd Bi At Ai Bt         ln A ln B 10.0 x Bi Ai        t Bi At Ai Bt 1.0 Bone Tissue 0.1 Below Above Iodine Image 20 20 80 80 100 100 120 120 40 40 60 60 K-edge Energy (keV) E K 33.17 50.24 ICTP School 2018 Giuliana Tromba 5

Phase – contrast imaging techniques: main cathegories Propagation-based Imaging (PBI) Analizer-Based Imaging (ABI) X-ray interferometry with crystals Grating interferometric imaging (GI) Grating non-interferometric imaging Exploiting the spatial coherence of SR … 6 ICTP School 2018 Giuliana Tromba

Phase Contrast vs. Absorption imaging  i , absorption term ,  phase shift term Ra diation – matter interaction is determined by refraction index : n = 1 - Absorption properties are expressed through  in the attenuation coefficient µ. The effect on phase of incident radiation produced by the sample (  or phase shift) is related to  for soft tissue@17 keV:     ,   ,  3 Absorption radiology -> contrast is generated by differences in the x-ray absorption ( C abs  x D, Phase Radiology -> contrast is generated by phase shifts ( C f  x D ) with x = object size // to beam direction >>   phase shifts effects >> absorption In conventional radiology image formation is based on differences in X-ray absorption properties of the samples (  term). The image contrast is generated by density, composition or thickness variation of the sample. Main limitation: poor contrast in soft tissue differentiation. Phase contrast techniques are based on the observation of the phase shifts produced by the object on the incoming wave (  term). Contrast arises from interference among parts of the wave front differently deviated (or phase shifted) by the sample. Edge enhancement effects. Incoming Transmitted wave wave sample sample a Incoming wave Distorted wave x Interference x x 10  a  100 z µ rad y z Intensity y Absorption Phase Contrast ICTP School 2018 Giuliana Tromba 7

Propagation based imaging (PBI) z R.Fitzgerard, Physics T oday, July 2000 • The technique exploits the high spatial coherence of the X-ray source. • z =0 -> absorption image • For z > 0 -> interference between diffracted and un-diffracted wave produces edge and contrast enhancement. A variation of  is detected Measure of  2  (x,y) • • The technique requires a high spatial coherence source, monochromaticity is not needed Regimes Snigirev A. et al., Rev. Sci. Instrum. 66, 1995 Wilkins S. W. et al., Nature 384, 1996 Cloetens P. et al., J. Phys D: Appl. Phys. 29, 1996 Arfelli F et al., Phys. Med. Biol. 43,1998 8 ICTP School 2018 Giuliana Tromba

Analyzer Based Imaging (ABI) R.Fitzgerard, Physics T oday, July 2000 • A perfect crystal is used as an angular filter to select angular emission of X-rays. The filtering function is the rocking curve (FWHM: 1-20  rad) Image formation with ABI is sensitive to a variation of  in the sample. Indeed, • refraction angle is roughly proportional to the gradient of  • Analyzer and monochromator aligned -> X-ray scattered by more than some tens µrad are rejected • Small misalignments -> investigation of phase shift effects • With greater misalignments the primary beam is almost totally rejected and pure refraction images are obtained Sensitive to  (x,y) • • The technique requires the beam monochromaticity. Podurets K. M. et al., Sov. Phys. Tech. Phys. 34(6), 1989 V. N. Ingal and E. A. Beliaevskaya, J. Phys. D: Appl. Phys. 28, 1995 Chapman D et al., Phys. Med. Biol. 42, 1997 9 ICTP School 2018 Giuliana Tromba

ABI image manipulation (original algorithm) I I 0,6 L H   0,5 1 /I 0 ] L H normalized intensity [I 0,4 0,3 0,2 0,1 6,726 6,728 6,730 6,732 6,734 angle [degree]      R       D   I I R  Linear approximation of rocking = refraction Image   L R L L z   Z curve at half values (I R and I L ) I    = apparent absorption   R   R     D   I I R image   H R H H z   (absorption+extinction) dR dR    I I   L H d d    I H L R     dR dR      R R   L H d d   H L          I R I R Apparent Absorption   H L L H Z dR dR Image    I I Refraction Image   L H d d   H L Ref: Chapman et al, Phys.Med.Biol, 42,1997 10 ICTP School 2018 Giuliana Tromba

Limitations and Requirements PBI • It is the simplest method as it requires the detector to be set at a certain distance from the sample. It does not require monochromaticity. • Requirements: • a high spatial coherence of the beam • adequate spatial resolution of the detector to detect interference fringes (edge- enhancement) • Exposure time related to beam intensity • The recorded signal is proportional to the second derivative of the phase term (  2  (x,y)) • Adequate to study samples with important variations of refractive index ABI • It requires the implementation and control of at least one crystal • Requirements: – high monochromaticity – parallel beam • Sensitive to beam instabilities • The recorded signal is proportional to the first derivative of the phase term (  (x,y)) • Adequate to study cartilages, joints, samples with wide variation of refractive intex ICTP School 2018 Giuliana Tromba

Interferometry : from phase shift to image contrast • Interferometry is a family of techniques in which waves are superimposed in order to extract information. • Widely used in optics (visible light) • It can be used in X-ray phase contrast imaging to transform the phase shift introduced by the object into image contrast • Two different interferometric approaches: • Crystal interferometry (Bonse and Hart, 1965) • Grating interferometry (David et al, 2002; Momose et al., 2003) Bonse, U. and Hart, M. (1965). Appl. Phys. Lett. 6 , 155 – 156. David, C., Nöhammer, B. et al. (2002). Appl. Phys. Lett. 81, 3287 – 3289 Momose, A. et al. (2003). Japan J. Appl. Phys.: 2 Lett. 42, L866 – L868 ICTP School 2018 Giuliana Tromba